Type alias vs type definition in Go

Question

I have stumbled upon this type alias in code:

type LightSource = struct {
  R, G, B, L float32
  X, Y, Z, A float32
  //...
}

My question is: what would be the reason to use a type alias like that to define a struct, rather than doing this?

type LightSource struct {
  R, G, B, L float32
  //...etc
}

Answer 1

---

Difference

• With =, it's alias type. It doesn't create a new type, it just creates a symbol to the same type.
• Without =, it's type definition. It creates a new type.

---

In usage

• When assign value of original type:
  • alias type can assign directly, without cast.
  • new type need cast, because it's a different type.
• Method on alias type can be used on original type; while method on type definition can't.

---

Example

type_alias_vs_new_test.go:

package main

import "testing"

type (
    A = int // alias type
    B int   // new type
)

func TestTypeAliasVsNew(t *testing.T) {
    var a A = 1 // A can actually be omitted,
    println(a)

    var b B = 2
    println(b)

    var ia int
    ia = a // no need cast,
    println(ia)

    var ib int
    ib = int(b) // need a cast,
    println(ib)
}